Production of devices based on electroluminescent display is a rapidly growing, billion dollar industry. Bright and efficient organic light-emitting diode (OLED) devices and electroluminescent (EL) devices have attracted considerable interest due to their potential application for flat panel displays (e.g., television and computer monitors). OLED based displays offer advantages over the traditional liquid crystal displays, such as: wide viewing angle, fast response, lower power consumption, and lower cost. However, several challenges still must be addressed before OLEDs become truly affordable and attractive replacements for liquid crystal based displays. To realize full color display applications, it is essential to have the three fundamental colors of red, green, and blue provided by emitters with sufficient color purity and sufficiently high emission efficiency.
In general, when a potential is applied across an OLED, holes are said to be injected from an anode into a hole transporting layer (HTL) while electrons are injected from a cathode into an electron transporting layer (ETL). The holes and electrons migrate to an ETL/HTL interface. Materials for these transporting layers are chosen so that holes are preferentially transported by the HTL, and electrons are preferentially transported by the ETL. At the ETL/HTL interface, the holes and electrons recombine to give excited molecules which radiatively relax, producing an EL emission that can range from blue to near-infrared (Koene, 1998).
In providing one of the key color components for electroluminescent display devices, blue luminescent compounds are among the most sought-after materials by industry around the world. Two alternative ways in which blue luminescence can be achieved are: (1) providing a molecule which emits blue color (emitter), and (2) doping an emitter such that the combination yields blue luminescence. Conveniently, the emitter can be an inorganic metal ion such as, for example, lanthanide, which emits blue light via d to f or f to f electronic transitions, or an organic molecule which has conjugated π bonds and emits blue light via π to π or π to n electronic transitions.
A common problem with blue emitters is their lack of long term stability in OLEDs. OLEDs generally suffer from a gradual intensity decrease of the blue hue, which results in gradual deterioration of the color purity of the display, and ultimately failure of the device. Television and computer monitors must perform consistently for at least five years in order to be commercially feasible. Even this modest expectation is a big challenge for currently available OLEDs.
There are several blue luminescent inorganic coordination compounds known (U.S. Pat. No. 6,500,569, U.S. Pat. No. 6,312,835, Yang, 2001, Jia et al., 2003); however, in some cases, due to a propensity for oxidation and/or hydrolysis reactions, such complexes are not very stable in solution. One family of known inorganic blue emitters, lanthanide ions, have low emission efficiency and require the use of a host (generally an inorganic salt), which makes it difficult to process them into thin films.
Thus, blue luminescent materials that are organic in nature are desirable due to their increased stability, solubility and ability to form thin films. A number of organic blue emitters are known to date (Shirota, 2000, Yang, 2001, Wu et al., 2001, and Liu et al., 2000). Many of these have poor luminescence efficiency and poor stability. Some are luminescent polymers that are difficult to apply in films using chemical vapor deposition (CVD) or vacuum deposition, processes known to produce superior films for electroluminescent displays. Even the best blue emitters currently available do not have the long term stability desired for commercial devices.
The limitations discussed above could restrict the market for OLED products, despite their many superior aspects as compared with liquid crystal displays. Therefore, in order for OLEDs to become truly feasible, there is a need for stable, organic emitters.